DESCRIPTION: (From the applicant's abstract) The process of antibody isotype switching and somatic hypermutation are important in the development of effective humoral responses. However, their understanding of the mechanisms involved in either isotype switching or somatic hypermutation is limited. The investigators have found that H-chain transgenes can undergo both isotype switching and somatic hypermutation. They have also found that a transgene construct can undergo VDJ sequence transfers that resemble gene conversion and correlate with somatic hypermutation. Surprisingly, analyses of both transgene isotype switching and "conversion" suggest that an elevated level of homologous recombination/gene conversion activity, which may have specificity for Ig gene sequences, is induced in some antigen-activated B cells. Their suggestion that homologous recombination/gene conversion plays a role in interchromosomal transgene isotype switching is based on an assumption that the Sm tandem repeat region is required for class switch recombination. One aim in this proposal will test this assumption; the Sm repeats will be deleted from the IgH locus in mice to ascertain Sm involvement in normal H-chain gene class switching. A second aim in this proposal will test the percentage of DNA homology require for transgene homologous recombination/gene conversion and the length of the DNA segments that are involved in the recombination/conversion process. The frequency of these events in differentiating B cells, and the phenotypes of the B cells undergoing the process, will also be investigated. The third aim of the proposal involves studies directed toward their suggestion that the homologous recombination/conversion process is targeted to antibody H-chain gene sequences. Several transgene constructs will be used to explore whether neighboring sequences affect the frequency of recombination/conversion between homologous gene segments. In addition, whether RNA transcription affects the frequency of recombination/conversion will also be investigated. The results from the proposed studies will demonstrate whether recombination/conversion activities are specifically induced in antigen-stimulated mouse B cells and will elucidate some of the important parameters that govern these processes. They believe that characterizations of these activities may lead to new insights into the poorly understood mechanisms of switch recombination and somatic hypermutation.